The invention relates to a method and apparatus for continuous digestion of wood chips, bagasse, and other finely divided fiber containing material. Conventional continuous digestors, such as shown in U.S. Pat. No. 3,380,883, provide for a number of treatment steps of a fiber column in the same vessel at various vertical positions in the vessel. Impregnation, cooking, and washing may all take place in the same vessel, and other steps, such as bleaching, may also take place, and the fiber material may be heated, cooled, diluted, dewatered, have chemicals added thereto, or otherwise be treated. In order for all such treatment steps in large volume to be properly carried out in a single vessel it is necessary to make the continuous digestor very tall. The great height of the fiber column and the liquid in the digestor, however result in large liquid pressure and fiber compaction. The fiber compaction may become so great that the free flow of liquid through the fiber column at the bottom portions thereof--which free flow is necessary for proper treatment of the fiber material--is prevented and so great that the evenness of the continuous fiber column movement is prevented. Also, the evenness of the liquid flow over the entire free cross-section of the digestor at the lower parts thereof--which even liquid flow is also necessary for proper treatment--may be prevented.
According to the present invention, a method and apparatus are provided that remedy the above-mentioned problems often existing in prior art digestors. In particular, according to the present invention, the compaction of fiber material in the lower portions of the digestor vessel is minimized so that free, even flow of liquid therethrough is permitted. This is accomplished by interrupting the fiber column of the vessel at one or more places along the height thereof, preferably at the transition areas between treatment zones, and by passing the fiber column through a liquid layer. For instance, in a continuous digestor having impregnating, cooking, and washing zones, the height of the fiber column is interrupted between the cooking and washing zones, and washing--while taking place vertically below the cooking zone--may be accomplished without interference due to compaction of fiber column. Also, since a new liquid layer is provided washing may take place with a higher dilution factor.
According to a preferred apparatus of the present invention, a table is provided at the transition area between various zones of a digestor, the table having formed around the periphery thereof an area in which the fiber material in the column may fall from the upper zone to the lower zone of the zones interrupted by the table. The table supports the weight of the fiber column above it, and a space is provided between the table and a fiber column which is re-formed in the zone below the table so that compaction does not occur. Liquid from the upper zone is removed near the table interface, and new liquid is introduced to the lower zone from beneath the table. Under normal circumstances the lower zone is vertically in line with the upper zone, however if preferred, the lower zone may be spaced from the vertical axis of the upper zone.
According to the present invention, the following advantages may be realized: Less fiber compaction, therefore better treatment (i.e. washing) of the fiber material. Ability to eliminate some screens from the treatment vessel, and thereby avoid problems of clogging, etc., that are often associated therewith. The ability to use less expensive materials in forming the lower zone compartments of the vessel, and to use less energy to pump the liquids from and between zones, etc.
The term "zone" as used in the specification is equivalent to the term "chamber", and these terms are used interchangeably.
It is the principal object of the present invention to provide an improved method and improved apparatus for continuous treatment of fiber material in a column. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.